Wireless cellular communication networks and their modes of operation are generally well known. In such a system the area covered by the network is divided into cells. Each cell is provided with a base station, which is arranged to communicate with a plurality of mobile stations or other user equipment in the cell associated with the base station.
In these known systems, a channel is typically allocated to each user. For example, in the case of the GSM (Global System for Mobile communications) standard, a user is allocated a given frequency band and a particular time slot in that frequency band. A single information stream from a single user can be allocated a frequency band and time slot. The so called Third Generation Standards currently being proposed use Code Division Multiple Access (CDMA). In this standard a user is allocated a particular spreading code to define a channel.
A method proposed to improve the capacity of a cellular system is one based on spatial diversity or spatial multiplexing. In this proposed system, the data rate can be increased by transmitting independent information streams from different antennas but using the same channel as defined by frequency, time slot and/or spreading code.
To successfully receive the different information streams, which are transmitted in parallel, it is necessary that the receiving end also have a number of antennas. In practice, for such a system to work, the number of propagation paths for each signal needs to be relatively high, that is, high enough to prevent the channel matrix H, the mathematical representation of the transmitted signal path through space, from being singular. Factors which affect the number of multiple paths, and therefore the transfer matrix, include the communications (radio) environment, the distance between the mobile and base stations and the positioning of the antennas for both the base and mobile stations.
These systems are also referred to as multiple input multiple output (MIMO) systems. These systems require complex controllers to control both the transmission and receiving elements of both the base station and the mobile station. The control circuitry needs to be able to control the antennas, the coding and the modulation of the signal at the transmission side, together with the control of the demodulation, and decoding at the receiving end of the communications link. This is especially complex where the communications equipment differs in capacity between base station to base station and user equipment to user equipment and from time to time.
One of the important factors in controlling a MIMO system is the control of the space-time code blocks or space time modulation system. Conventional modulation techniques such as Amplitude Modulation, Frequency Modulation and Phase modulation, are considered to be modulated at the transmitter with respect to time. The known Space-time modulation systems expand on this concept by creating a code comprising various transmittable symbols which are dependent not only on the time of transmission but also on the position of the individual antenna elements and directions in space formed by the combined use of multiple transmit antennas as they are transmitted.
Prior art space-time coding systems have been proposed. EP-0951091 describes a system generally known as a BLAST (Bell Laboratories layered Space Time architecture) modulation system. In such a system a conventional time modulated code such as Quadrature Phase Shift Keying (QPSK) is used, a group of symbols are created, stored, and each symbol transmitted from each of the transmission antenna in the antenna array. The receiving antenna array then receives and demodulates and decodes the transmitted symbols.
EP-0951091 describes a wireless communication system having a space-time architecture employing multi-element antennas at both the transmitter and receiver. Within this document is disclosed a method for possibly selecting a sub-set of the antennas to be used for transmission based on feed-back from the receiver. This document though does not address the problem of controlling a MIMO system, where users with different equipment and requirements can co-exist within the same system, and base stations may have different capabilities.
BLAST systems though rely on the number of receiver antenna in the receiver antenna array to be greater than the number of transmit antenna in the transmitter antenna array. Where this is not true the BLAST type modulation system is unable to reach the channel capacity.